The present invention relates to a conveying member for conveying a recording medium carrying a toner image transferred thereto from an image carrier, an image transferring device including the conveying member, and an image forming apparatus including the image transferring device.
An image forming apparatus of the type including a photoconductive element, intermediate image transfer belt or similar image carrier and a transfer belt, transfer drum or similar conveying member is conventional. A toner image formed on the photoconductive element is transferred to a sheet or similar recording medium by the conveying member to which a bias for image transfer is applied. In this type of apparatus, image transfer is effected by the electric resistance of the conveying member, e.g., transfer belt. Various approaches have heretofore been proposed to provide the transfer belt with an adequate resistance.
Japanese Patent Laid-Open Publication No. 63-83762, for example, teaches a transfer belt including a portion formed of a semiconductor material and having a volume resistivity of 10.sup.10 .OMEGA.cm to 10.sup.13 .OMEGA.cm. The transfer belt is passed over a drive roller and a ground roller spaced from each other by a preselected distance. A wrap roller supports the rear or inner surface of the belt in the vicinity of a photoconductive element. In this configuration, the portion of the belt between the ground roller or charge reduction source and the wrap roller plays the role of a stable resistor, so that charge fed from a transfer charger can be held in a stable condition. A technology relating to the electric resistance of the transfer belt is also disclosed in Japanese Patent Laid-Open Publication No. 1-121877.
Japanese Patent Laid-Open Publication No. 2-110586 discloses a transfer belt made up of a resistance layer having a resistance higher than 10.sup.14 .OMEGA.cm and close to a photoconductive element, and a resistance layer having a resistance lower than 10.sup.14 .OMEGA.cm and remote from the photoconductive element. With this structure, the transfer belt has its surface potential regulated to a desired gradient. This kind of structure is directed toward the obviation of the flying of toner and the local omission of an image.
Assume that a bias for image transfer is applied from the rear or inner surface of the transfer belt remote from the photoconductive element. Then, if the surface resistivity of the rear of the belt is low, a transfer current easily flows from a position where the bias is applied to the other region, as well known in the art. Usually, the transfer current flows to a nip between the photoconductive element and the belt and where image transfer is expected to occur. However, when the surface resistivity of the rear is low, the transfer current flows to a position upstream of the above nip, i.e., where the photoconductive element and belt do not contact each other. As a result, transfer charge flows out to the position upstream of the nip, forming an electric field. This electric field causes a toner image to be partly transferred from the photoconductive element to a sheet being conveyed by the belt. Such an occurrence is generally referred to as pretransfer. Because the pretransfer occurs at the position upstream of the nip or regular image transfer position, the above part of the toner image is transferred to the position of the sheet deviated from the expected position. Let this undesirable occurrence be referred to as toner scattering hereinafter.
The toner scattering is aggravated in a low humidity environment of the following reason. The surface resistivity of the transfer belt is higher when humidity is low than when it is normal. As a result, in a low humidity environment, the voltage on the surface of the belt increases in the region preceding the nip, causing discharge to occur between the photoconductive element and the belt.
Furthermore, if the surface resistivity of the front of the transfer belt facing the photoconductive element is excessively high, then the charge derived from the bias applied to the belt remains even after image transfer. Consequently, when image formation is repeated, images formed on the second sheet and successive sheets or on both sides of sheets are apt to be defective.
On the other hand, the surface resistivity of the transfer belt has influence on the conveyance of the sheet by the belt, and the separation of the sheet from the photoconductive element which is effected by the conveyance. Specifically, when the surface resistivity of the belt is low, charge great enough for the sheet to electrostatically adhere to the belt stably is not accumulated. In this condition, it is likely that the sheet slips on the belt and brings about the dislocation of the toner image or that the sheet moved away from the nip wraps around the photoconductive element without being separated from the element. Moreover, in a high humidity environment, the surface resistivity of the belt is higher than in a normal humidity environment and causes the charge on the belt to reduce. This reduces the electrostatic adhesion of the sheet to the belt and thereby aggravates the defective separation of the sheet from the photoconductive element.
Although various schemes relating to the resistance of the transfer belt have been proposed in the past, as stated earlier, all of them regulate the resistance of the belt in the macroscale in terms of, e.g., volume resistivity. Stated another way, none of the conventional schemes clear up the mechanism relating to the image transfer and the conveyance of the sheet, i.e., the separation of the sheet from the photoconductive element.